Improvements on Fresnel arrays for high contrast imaging

TL;DR

This paper discusses advancements in Fresnel array design for high contrast UV imaging, aiming to enable space-based observations of faint astrophysical sources near bright objects, with plans for a small prototype on the ISS.

Contribution

It introduces improved Fresnel array designs and optical correction methods to enhance UV imaging performance for space telescopes.

Findings

Improved Point Spread Function in UV through array design optimization

Enhanced dynamic range using combined apodization and mesh optimization

Numerical simulations demonstrate significant performance gains

Abstract

The Fresnel Diffractive Array Imager (FDAI) is based on a new optical concept for space telescopes, developed at Institut de Recherche en Astrophysique et Plan\'etologie (IRAP), Toulouse, France. For the visible and near-infrared it has already proven its performances in resolution and dynamic range. We propose it now for astrophysical applications in the ultraviolet with apertures from 6 to 30 meters, aimed at imaging in UV faint astrophysical sources close to bright ones, as well as other applications requiring high dynamic range. Of course the project needs first a probatory mission at small aperture to validate the concept in space. In collaboration with institutes in Spain and Russia, we will propose to board a small prototype of Fresnel imager on the International Space Station (ISS), with a program combining technical tests and astrophysical targets. The spectral domain should contain the Lyman-$\alpha$ line ($\lambda= 121$ nm). As part of its preparation, we improve the Fresnel array design for a better Point Spread Function in UV, presently on a small laboratory prototype working at 260 nm. Moreover, we plan to validate a new optical design and chromatic correction adapted to UV. In this article we present the results of numerical propagations showing the improvement in dynamic range obtained by combining and adapting three methods : central obturation, optimization of the bars mesh holding the Fresnel rings, and orthogonal apodization. We briefly present the proposed astrophysical program of a probatory mission with such UV optics. Keywords: Fresnel arrays -- Diffractive optics -- UV imaging -- Apodization -- High dynamic range -- High angular resolution -- Exoplanets